Appendix l-D— The Impact of Genetics on Ethanol— A Case Study • 297 
sidiuil lignin, which is remoxed hetore (bv sohents 
extraction) or alter In clroh sis, is used to pi o\ ide 
energy for ethanol recoxerv. Kxtensixe xvork on this 
approach has been done at the I'nix ersity of Califor- 
nia, Berkelex , and the I'.S. .Army Xatick Laboratories. 
In scheme B. the cellulase is not recoxered but 
rather, the xx hole fermentation broth from cellulase 
production is added to the cellulosic biomass along 
xx ith ethanol-[)roducing yeast. The result is a simul- 
taneous cellulose hydrolysis (saccharification) and 
fermentation. (In the [)roduction of ethanol from 
starch, the starch hydrolyzing enzymes are added at 
the same time as the yeast for simultaneous sacchari- 
fication and fermentation.) I'his technologx' has been 
demonstrated hx the Culf Oil Co. After fermentation, 
the ethanol is recoxered and the residual lignin can 
again he used for energx for distillation. The prob- 
lem of unused pentose sugar still remains and xx ill re- 
quire a separate fermentation step. 
A third alternatix e (scheme C, figure l-D-3) shoxx s a 
simpler approach, nameix a direct fermentation on 
cellulose. I'his approach has been dexeloped at the 
Massachusetts Institute of Technology'. It utilizes 
bacteria that xx ill produce cellulase to hydrolyze the 
cellulose and hemicellulose and ferment both the 
hexose and pentose sugars to ethanol in a single- 
stage reactor. The adxantage of this approach is a 
minimal requirement for pretreatment, a combined 
enzyme production, cellulose hx drolysis and ethanol 
fermentation, and simultaneous conxersion of both 
pentose and hexose sugars to ethanol. This concept 
is nexx and xx ork still needs to he done to increase the 
ethanol concentration, minimize side product forma- 
tion, and increase the rate of ethanol production. 
.Again, residual lignin xxill be used to proxide the 
energy for ethanol distillation. 
FERME.NT.ATION OF ETHAXOL 
.An examination of the economics for ethanol pro- 
duction shoxx s that the dominant cost is the process 
raxx material. .As seen in table I-D-2 the feedstock rep- 
resents 60 to 70 percent of the manufacturing cost. 
Thus, it is clear that any improxement in substrate 
utilization efficiency is of substantia] benefit. The 
theoretical yields of ethanol from glucose, sucrose, 
and starch or cellulose are 0.51, 0.54 and 0.57 gram 
(g) ethanol'g material, respectixely; the differences 
result from the addition of a molecule of xvater on 
hydrolysis. There are sex eral approaches to improve 
the yield abox e the typical value of 90 to 95 percent 
currently achiex ed. These are: 
• increase the ratio of ethanol produced per unit 
weight of cells, e.g., through cell recycle, 
vacuum fermentation, immobilized cells, or im- 
proxement in specific productix ity (g ethanol/g 
Table I-D-2.— A Comparison of the Distribution of 
Manufacturing Costs for Several Ethanol 
Production Processes 
Substrate 
ivlolasses 
Corn 
Grain 
Sorghum 
Cost component (%) 
Capital 
9 
12 
10 
Operating 
20 
26 
30 
Feedstock 
71 
62 
60 
Total 
100 
100 
100 
Cost on energy basis 
(SMiVIBtu) 
12.5 
14.9 
12.7 
Cost/gal etiianol ($/gal) . . . 
1.05 
1.25 
1.07 
Capital investment 
($/annual gal) 
1.02 
1.05 
1.75 
SOURCE: "Comparative Economic Assessment of Ethanol From Biomass,” 
Mitre Corp., report HCP/ET-2854). 
cell hr), by increasing the content and/or activi- 
ty of those enzymes in the pathway to ethanol; 
• increase the utilization of other materials in the 
substrate, e.g., the use of oligosaccharides, espe- 
cially branched, in starch, and the use of con- 
taminating sugars such as galactose or mannose 
for hemicellulose; and 
• dex elop a route for the utilization of pentose su- 
gars, especially xylose, present in hemicellulose. 
The potential effect of oligosaccharides or con- 
taminating sugar utilization is relatively small, since 
they represent typically 1 to 3 percent of the total 
sugar content. Hoxvex'er, if cellulosic biomass con- 
taining 15 to 25 percent hemicellulose is used, then 
the impact of pentose conversion to ethanol is great. 
Cellulosic biomass is made up primarily of cellu- 
lose, hemicellulose (mostly xylan) and lignin. Other 
components such as protein, ash, fats, etc., typically 
comprise about 10 percent. The composition of l-io- 
mass can be expressed in terms of the following 
equation: 
'-L\ 
where F^, F„, Fl, and F^ are the weight fractions of 
cellulose, hemicellulose, lignin, and ash, respectively. 
Assuming that the ash is 10 percent (F^ = 0.1) and 
that Fj. and F„ are the only fermentable components 
in the biomass, then: 
Fc = Fh = 0.9 - Fl (2) 
The maximum amount of ethanol from one unit of 
biomass (’Ve,b) is: 
~ ^E/H^H ~ '^E/B 
Where and Ye,h are the yield of ethanol for cel- 
lulose and hemicellulose, respectively. Equation 2 
can be rearranged to relate the fractions of cellulose: 
